1. Field of the Invention
The present invention relates to an optical image measuring apparatus that applies a light beam to an object to be measured, particularly a light scattering medium, and produces a surface form or inner form of the object to be measured by detecting a reflected light beam or a transmitted light beam. In particular, the present invention relates to an optical image measuring apparatus for measuring the surface form or inner form of the object to be measured by using an optical heterodyne detection method to produce the image of the measured form.
2. Description of the Related Art
In recent years, attention has been given to optical imaging technique that produces an image of a surface or inner portion of an object to be measured using a laser light source or the like. This optical imaging technique is not hazardous to human bodies in contrast to the conventional X-ray CT. Therefore, the development of applications in the medical field has been particularly expected.
An example of a typical method of the optical imaging technique is a low coherent interference method (also called ‘optical coherence tomography’ or the like). This method uses the low coherence of a broad-band light source having a broad spectral width, such as a super luminescent diode (SLD). According to this method, reflection light from an object to be measured or light transmitted therethrough can be detected at superior distance resolution on the order of μm (for example, see Naohiro Tanno, “Kogaku” (Japanese Journal of Optics), Volume 28, No. 3, 116 (1999)).
FIG. 8 shows a basic structure of a conventional optical image measuring apparatus based on a Michelson interferometer, as an example of an apparatus using the low coherent interference method. An optical image measuring apparatus 200 includes a broad-band light source 201, a mirror 202, a beam splitter 203, and a photo detector 204. An object to be measured 205 is made of a scattering medium. A light beam from the broad-band light source 201 is divided by the beam splitter 203 into two, that is, reference light R propagating to the mirror 202 and signal light S propagating to the object to be measured 205. The reference light R is light reflected by the beam splitter 203. The signal light S is light transmitted through the beam splitter 203.
Here, as shown in FIG. 8, a propagating direction of the signal light S is set as a z-axis direction and a plane orthogonal to the propagating direction of the signal light S is defined as an x-y plane. The mirror 202 is movable in a direction indicated by a double-headed arrow in FIG. 8 (z-scanning direction).
The reference light R is subjected to a Doppler frequency shift through when reflected by the z-scanning mirror 202. On the other hand, the signal light S is reflected from the surface of the object to be measured 205 and from the inner layers thereof when the object to be measured 205 is irradiated with the signal light S. The object to be measured 205 is made of the scattering medium, so reflection light of the signal light S may be a diffusing wave having random phases. The signal light propagating through the object to be measured 205 and the reference light that is reflected by the mirror 202 to be subjected to the frequency shift are superimposed on each other by the beam splitter 203 to produce interference light.
In the image measurement using such a low coherent interference method, interference occurs only when a difference in optical path length between the signal light S and the reference light R is within the coherence length (coherent distance) on the order of μm of the light source. In addition, only the component of the signal light S whose phase is correlated to that of the reference light R interferes with the reference light R. That is, only the coherent signal light component of the signal light S selectively interferes with the reference light R. Based on their principles, the position of the mirror 202 is moved by the z-scanning to vary the optical path length of the reference light R, so that a reflectance profile of the inner layers of the object to be measured 205 is measured. The object to be measured 205 is also scanned with the irradiated signal light S in an x-y plane direction. The interference light is detected by the photo detector 204 during such scanning in the z-direction and the x-y plane direction. An electrical signal (heterodyne signal) outputted as a detection result is analyzed to obtain a two-dimensional sectional image of the object to be measured 205 (see Naohiro Tanno, “Kogaku” (Japanese Journal of Optics), Volume 28, No. 3, 116 (1999)).
Assume that an intensity of the reference light R and an intensity of the signal light S which are superimposed by the beam splitter 203 are given by Ir and Is, respectively, and a frequency difference between the reference light R and the signal light S and a phase difference therebetween are given by fif and Δθ, respectively. In this case, a heterodyne signal as expressed by the following expression is outputted from the photo detector (for example, see Yoshizawa and Seta “Optical Heterodyne Technology (revised edition)”, New Technology Communications (2003), p. 2).
Expression (1)i(t)∝Ir+Is+2√{square root over (IrIs)} cos(2πfift+Δθ)  (1)
The third term of the right side of the expression (1) indicates an alternating current electrical signal and the frequency fif thereof is equal to the frequency of beat caused from the interference between the reference light R and the signal light S. The frequency fif of an alternating current component of the heterodyne signal is called a beat frequency or the like. The first and second terms of the right side of the expression (1) indicate the direct current components of the heterodyne signal and correspond to a signal intensity of background light of interference light.
However, when the two-dimensional cross sectional image is obtained by the conventional low coherent interference method, it is necessary to scan the object to be measured 205 with a light beam and to successively detect reflection light waves from respective regions of the object to be measured 205 in a depth direction (z-direction) and a sectional direction (x-y plane direction). Therefore, the measurement of the object to be measured 205 requires a long time. In addition, it is hard to shorten a measurement time in view of measurement fundamentals.
In views of such problems, an optical image measuring apparatus for shortening a measurement time has been proposed. FIG. 9 shows a fundamental structure of an example of such an apparatus. As shown in FIG. 9, an optical image measuring apparatus 300 includes a broad-band light source 301, a mirror 302, a beam splitter 303, a two-dimensional photo sensor array 304 serving as a photo detector, and lenses 306 and 307. A light beam emitted from the light source 301 is converted into a parallel light flux by the lenses 306 and 307 and a beam diameter thereof is widened thereby. Then, the parallel light flux is divided into two, that is, the reference light R and the signal light S by the beam splitter 303. The reference light R is subjected to Doppler frequency shift through z-scanning with the mirror 302. On the other hand, the signal light S is incident on the object to be measured 305 over a broad area of the x-y plane because the beam diameter is widened. Therefore, the signal light S becomes reflection light including information related to the surface and inner portion of the object to be measured 305 over a wide area. The reference light R and the signal light S are superimposed on each other by the beam splitter 303 and detected by elements (photo sensors) arranged in parallel on the two-dimensional photo sensor array 304. Thus, it is possible to obtain a two-dimensional cross sectional image of the object to be measured 305 in real time without light beam scanning.
An apparatus described in K. P. Chan, M. Yamada, and H. Inaba, “Electronics Letters”, Vol. 30, 1753 (1994) has been known as such a non-scanning type optical image measuring apparatus. In the apparatus described in the same document, a plurality of heterodyne signals outputted from a two-dimensional photo sensor array are inputted to signal processing systems arranged in parallel to detect the amplitude and phase of each of the heterodyne signals.
However, when the spatial resolution of an image is increased, it is necessary to increase a number of elements of the array. In addition, it is necessary to prepare a signal processing system including a number of channels corresponding to the number of elements. Therefore, it is supposedly hard to actually use the apparatus in fields that require a high resolution image, such as a medical field and an industrial field.
Thus, the inventors of the present invention have proposed the following non-scanning type optical image measuring apparatus in JP 2001-330558 A (claims and specification paragraphs [0044] and [0072] to [0077]). The optical image measuring apparatus according to this proposal includes a light source for emitting a light beam, an interference optical system, and a signal processing portion. In the interference optical system, the light beam emitted from the light source is divided into two, that is, signal light propagating through an examined object arrangement position in which an object to be examined is arranged and reference light propagating on an optical path different from an optical path passing through the examined object arrangement position. The signal light propagating through the examined object arrangement position and the reference light propagating on the different optical path are superimposed on each other to produce interference light. The interference optical system includes a frequency shifter, light cutoff devices, and photo sensors. The frequency shifter shifts a frequency of the signal light and a frequency of the reference light relative to each other. In order to receive the interference light in the interference optical system, the interference light is divided into two parts. The light cutoff devices periodically cut off the two divided parts of the interference light to generate two interference light pulse trains with a phase difference of 90 degrees therebetween. The photo sensors respectively receive the two interference light pulse trains. The photo sensors each have a plurality of light receiving elements which are spatially arranged and separately detect light receiving signals. The signal processing portion combines the plurality of light receiving signals detected by the photo sensors to generate signals of the signal light which correspond to respective points of interest of a surface or inner layers of the object to be examined which is arranged in the examined object arrangement position on a propagating path of the signal light.
In the optical image measuring apparatus, the interference light in which the reference light and the signal light interfere with each other is divided into two parts. The two parts of the interference light are received by the two photo sensors (two-dimensional photo sensor arrays) and respectively sampled by the light cutoff devices (shutters) disposed in front of both sensor arrays. A phase difference of π/2 is set between sampling periods of the two divided parts of the interference light. Therefore, an intensity of the signal light and an intensity of the reference light which compose background light of the interference light and phase quadrature components (sine component and cosine component) of the interference light are detected. In addition, an intensity of the background light included in outputs from both the sensor arrays is subtracted from the outputs of both the sensor arrays to calculate two phase quadrature components of the interference light. An amplitude of the interference light is obtained based on the calculation result.
An available image sensor such as a charge-coupled device (CCD) camera has been widely used for the two-dimensional photo sensor array of the optical image measuring apparatus as described above. However, up to now, a problem has been recognized that a currently available CCD camera cannot follow the beat frequency of a heterodyne signal which is the order of several kHz to several MHz because of the low frequency response characteristic thereof. The feature of the optical image measuring apparatus which is proposed by the inventors of the present invention and described in JP 2001-330558 A (claims, specification paragraphs [0068] to [0084], and FIG. 1) is to perform the measurement using the low frequency response characteristic based on the sufficient recognition of the problem.
In the optical image measuring apparatus described in JP 2001-330558 A (claims, specification paragraphs [0068] to [0084], and FIG. 1), the acquisition of one frame of an x-y tomographic image takes a time of about one millisecond. In order to form a three-dimensional image of the object to be measured, a tomographic image thereof (x-z tomographic image or y-z tomographic image) in a measurement depth direction, or a tomographic image thereof in a direction oblique to an axis, it is necessary to acquire x-y tomographic images at many measurement depths (z-coordinates). In order to acquire, for example, the three-dimensional image of the object to be measured with suitable precision, it is necessary to perform scanning in a measurement depth direction (z-direction) at a predetermined interval of, for example, about 5 micrometers. For example, the three-dimensional image of the object to be measured is formed on the assumption that the respective x-y tomographic images are measured at the predetermined interval.
The formation of the three-dimensional image or the like takes a time of one or more seconds depending on, for example, a type of object to be measured. However, when a movable object to be measured, such as a human eye which is a living tissue moves during the measurement of the object to be measured, it is likely to cause the displacement of the measurement position related to each of the x-y tomographic images to reduce the precision of the three-dimensional image or the like.
In particular, when the measurement interval for the x-y tomographic images in the measurement depth direction is displaced, the above-mentioned assumption related to the formation of the three-dimensional image or the like is not satisfied, so the precision of the image significantly reduces. Therefore, profile of some kind for associating the x-y tomographic images in the measurement depth direction (z-direction) with one another is required.
With respect to a method of acquiring the information of the object to be measured in the z-direction, there has been known, for example, an optical measurement apparatus as shown in FIG. 10 (for example, see A. F. Fercher, C. K. Hitzenberger, G. Kamp, and S. Y. Elzaiat, “Optics Communication”, Vol. 117, pp. 43-48 (1995)). In an optical measurement apparatus 400 shown in FIG. 10, a light beam from a broad-band light source 401 is divided by a half mirror 402 into signal light propagating to an object to be measured 404 and reference light propagating to a mirror (fixed mirror) 403 which is fixedly disposed. The signal light reflected on the object to be measured 404 and the reference light reflected on the fixed mirror 403 are superimposed on each other to produce interference light. The interference light is separated into wavelength components having wavelengths λ1 to λn by a diffraction grating 405. The respective wavelength components are detected by a one-dimensional photo sensor array 406. Each of photo sensors composing the one-dimensional photo sensor array 406 outputs a detection signal indicating a light intensity of each of the detected wavelength components to a computer 407.
The computer 407 acquires a relationship between a wavelength and a light intensity of the interference light, that is, an light intensity distribution (wavelength spectrum) of the interference light, based on the detection signals of the respective wavelength components of the interference light which are outputted from the one-dimensional photo sensor array 406. FIG. 11A is a schematic graph showing an example of the wavelength spectrum of the interference light.
Then, the computer 407 performs Fourier transform on the acquired wavelength spectrum of the interference light. As a result, as shown in FIG. 11B, an interference signal intensity distribution based on the z-coordinate (measurement depth) of the object to be measured 404 as a variable is acquired. This is information depending on the measurement depth of the object to be measured 400. The one-dimensional photo sensor array 406 normally has a readout rate of 1 MHz or more (that is, 1μ seconds or less). Therefore, the interference signal intensity distribution based on the measurement depth as a variable can be acquired at the same rate.